When objects have to be weighed with high precision, one needs to minimize as much as possible all factors that have an influence on the weighing result. This includes for example air drafts which can in most cases be reduced sufficiently by means of a draft shield so that they can be ignored, or fluctuations of the ambient conditions, including temperature and humidity, which can be eliminated for example by placing the balance in a climate-controlled environment.
Another factor affecting the weighing result is the presence of electrostatic charges in the weighing compartment, specifically in the vicinity of, or directly on, the weighing object, which manifest themselves particularly when samples are weighed that are non-conductive or are contained in vessels of non-conductive material. The effect of electrostatic charges poses a big problem in particular when large glass- or polymer containers are used in situations where the weighing task involves the determination of small mass differences. In most cases, there is no build-up of electrostatic charges on the load receiver itself on which the samples and/or containers that make up the weighing load are placed, since the load receiver is normally made of metal, and the charges can drain off through a conductive connection to the electrically grounded balance housing.
A build-up of electrostatic charges on the weighing load on the load receiver gives rise to a difference in the respective electrostatic potentials of the weighing load and the grounded parts of the balance. Due to this difference in electrostatic potentials, there will be a force acting between the weighing load and, e.g., the floor of the weighing compartment. The vertical component of this force will also act on the weighing cell, in addition to the weight force generated by the weighing object. Consequently, an incorrect weighing result will be produced and displayed.
For example when a sample is weighed in a plastic vessel, electrostatic charges can cause a measurement error of 2 milligrams. For a sample of 20 milligrams, the measurement error can thus be as high as 10% of the weight of the sample.
Other balance parts such as for example an added draft shield or other areas of the housing, to the extent that they consist of non-conductive materials, can likewise accumulate electrostatic charges and introduce errors in the weighing result through a so-called force bypass.
An analytical balance designed to weigh electrostatically charged weighing objects is described in EP 1 106 978 A1. The balance, which has a draft shield, is equipped with a device for generating a stream of ionized air. This device includes an air ionizer with at least two pinpoint electrodes held at respective voltages of opposite polarity, as well as a fan. The device can be activated by the closing of the draft shield access openings and can be turned off after a preset time period has elapsed.
This device has on the one hand the disadvantage that the fan can produce unwanted air drafts in the area of the load receiver, and on the other hand that there is always a fixed amount of ionized air molecules available to neutralize the electrostatic charges, regardless of the magnitude of the electrostatic charge accumulated by the object on the load receiver.
A balance with an electrostatic charge sensor to detect the presence of electrostatic charges and to provide a signal representative of the amount of the built-up charges is disclosed in EP 1 813 920 A1. The balance, which includes a load receiver for the weighing object and an electronic part with a measuring module, is equipped with an electrostatic sensor that is arranged in such a way that electrostatic charges on an object that is placed, or about to be placed, on the load receiver can be detected.
While the spread of electrostatic charges is continuous, they are not uniformly distributed in space. Their concentration and polarity are different for each material. This means that the charges detected with the arrangement of EP 1 813 920 A1 are predominantly in the vicinity of the electrostatic sensor. The measurement result of the electrostatic sensor depends on the specific location where the sensor is located in the balance. The information gained from the measurement of electrostatic charges with an electrostatic sensor therefore has only limited validity.
As a further disadvantage, the electrostatic sensor measures the magnitude of the electrostatic charge independently of the direction of the forces originating from them. In a balance, only the vertical components of the electrostatic attraction forces have an influence on the weighing result, and it is also essential in which direction the vertical force pulls the weighing object, i.e. upward or downward. The measurement of the total electrostatic charge that is present in the vicinity of the electrostatic sensor is therefore irrelevant.
Furthermore, this balance requires the use of an additional sensor besides the weighing cell.
It is therefore the object of the present invention to propose a method and a device which will make it possible to estimate the extent to which the weighing result of a balance is influenced by electrostatic charges that are present on a weighing object. The aim is for a solution with a minimum of additional electronic circuitry.